Recently, active studies have been conducted to develop a substitute for the existing fossil fuels in order to reduce emission of carbon dioxide regarded as a main cause of global warming and to solve the energy-related problems at hand. Intensive studies about the use of natural energy sources, such as wind, atomic force and solar light, have been made to allow those energy sources to substitute for petroleum sources that may be exhausted in the near future. Among those energy sources, solar cells using solar energy are the most eco-friendly and are inexhaustible, unlike the other energy sources, with the proviso that adequate methods of utilizing solar energy are developed. Since solar cells using selenium (Se) were developed in 1983, silicon solar cells have been spotlighted more recently. However, silicon solar cells require high manufacturing cost and thus are not commercially practical, and are insufficient in terms of cell efficiency. To overcome such problems, many workers have studied intensively to develop economical dye-sensitised solar cells.
While organic light emitting displays (OLEDs) use electric energy as their driving mechanism, dye-sensitised solar cells are based on the mechanism in which they absorb light energy in the visible range to produce electron-hole pairs. In addition, dye-sensitised solar cells are photoelectrochemical solar cells including photosensitive dye molecules and transition metal oxides transporting the resultant electrons as their main constitutional elements. Typical examples of such dye-sensitised solar cells include those using titanium dioxide nanoparticles, which are developed in 1991 by Michael Graetzel and coworkers of Ecole Polytechnique Federale de Lausanne (EPFL).
The above dye-sensitised solar cells developed by Michael Graetzel and coworkers are advantageous in that they may be applied to outer wall windows of buildings and glass greenhouses due to their transparent electrodes and they are cost-efficient as compared to the existing silicon solar cells. However, such solar cells have low photoelectric current conversion efficiency, and thus may not be commercially practical.
Photoelectric current conversion efficiency is in proportion to the amount of electrons generated by the absorption of solar light. In order to increase the efficiency of a solar cell, electron generation may be increased by increasing solar light absorption or the amount of dye adsorption, or by preventing loss of the excited electrons caused by electron-hole recombination. It is possible to increase the reflectivity of a platinum electrode in order to increase solar light absorbance. It is also possible to provide oxide semiconductor particles with a nano-scaled size in order to increase the dye adsorption per unit area. Addition of a semiconductor oxide light scattering agent having a size of several micrometers is also known in the art.
However, improvement in photoelectric current conversion efficiency is still limited in the above-mentioned methods. Therefore, there is an imminent need for developing novel technologies to improve photoelectric current conversion efficiency, in particular, for developing a novel dye having high light absorbance as well as a broad range of light absorption to improve photoelectric current conversion efficiency.